Many electrostatic copiers and laser printers employ removable sheet-holding trays which, when inserted, enable automatic feeding of sheets from a stack held in a tray. In general, sheets are fed in the direction of insertion of the tray into the printer/copier. The sheet pick and feed mechanism is integral to the printer/copier and engages the stack of sheets when the tray is inserted.
A popular mechanism for paper picking and feeding employs corner separation devices in conjunction with "D" shaped feed rollers. The D rollers attempt to feed a top sheet from a stack but the corners of the sheet are trapped under metal corners separators. As the D rollers rotate and continue to attempt to feed the sheet, the sheet slides and buckles at the corners until it snaps out from under the corner separators and is free to continue travelling into the printer/copier. The rest of the stack is held in position by the corner separators.
The corner separation method is popular as it is relatively simple and inexpensive. It's principal disadvantage is, however, that it is not a reliable method for separating sheets of paper, especially when the properties of the paper vary with humidity, handling, etc. With certain papers, the amount of friction between individual sheets in a stack can vary greatly. In such case, the corner separation mechanism is unable to separate a first sheet from the rest of the stack, and two or more sheets of paper are fed into the printer, potentially causing a jam.
Sheet feed trays that incorporate corner separators also often include springs and plates to push the stack of sheets up and into engagement with a sheet pick mechanism. In addition to being noisy, such springs and plates render it more difficult to load paper into the tray, and, at times, cause the paper to be loaded improperly. Furthermore, such springs and plates are often engaged by solenoids and cams which add cost and complexity to the unit.
U.S. Pat. Nos. 5,199,696 and 5,039,080, both to Kato, illustrate a sheet feeding unit similar to that described above. More particularly, Kato describes a paper feeding unit wherein a stack of sheets are held in a cassette that is insertable into a copier mechanism. Sheet feed is from the rear of the cassette, with the stack of sheets held on a movable plate which raises the stack into engagement with a pick roller. The pick roller is rotatable so as to be either in engagement with or out of engagement with the stack of sheets, depending upon the status of the copier. Counter-rotating rollers are used to enable sheet separation during the feed action.
Accordingly, it is an object of this invention to provide an improved sheet feed mechanism that avoids the use of corner separators to enable sheet separation.
It is another object of this invention to provide an improved sheet feed mechanism which enables sheet feed in a direction of paper tray extraction.
It is yet another object of this invention to provide an improved sheet feed mechanism for cooperation with a stack of sheets in a removable sheet-holding tray, wherein the tray includes no pressure plate or other stack-movement mechanism.